An optical signal for use in an optical communication system is transmitted in an optical fiber laid down over a long distance. Hence, as the characteristic of a semiconductor laser used as a light source of the light, high output and high stability are required.
When a single mode optical fiber is used in transmission of an optical signal, the semiconductor laser as a light source and the single mode optical fiber are coupled optically.
However, when a device end face of the semiconductor laser and a light incident plane of the optical fiber are directly coupled by means of butt joint, the light wave spot size of the semiconductor laser is 2 to 3 μm while the spot size of the single mode optical fiber is about 10 μm, so that coupling loss occurs due to difference in light wave spot size between mutual optical waveguides.
Generally, therefore, it is attempted to reduce the coupling loss by converting the laser light from the semiconductor laser into the spot size of the optical fiber by using a lens.
In the configuration using a lens in optical coupling, however, it requires complicated optical axis adjustment between the semiconductor laser and the optical fiber, and the number of optical components is increased.
To solve these problems, there has been proposed a semiconductor optical device capable of coupling optically with a single mode optical fiber by using a lens as disclosed in the following patent document 1.
FIG. 17 is a perspective view showing a schematic configuration of the semiconductor optical device disclosed in patent document 1.
As shown in FIG. 17, a semiconductor optical device 51 disclosed in patent document 1 comprises components 52 necessary for forming devices such as a cladding, a cap and an electrode, in which a multiquantum well (MQW) active layer 53 is coupled to a taper optical waveguide 55 by way of a butt joint part 54.
The MQW active layer 53 is a distortion super-grating active layer, which composes an active region 56. The light waveguide 55 is a light waveguide whose layer thickness and band gap wavelength are continuously changed, and composes a spot size conversion region 57.
In the case of coupling optically with a single mode optical fiber by using the semiconductor optical device 51, laser light emitted from the active region 56 of the MQW active layer 53 is propagated to the spot size conversion region 57 of the taper light waveguide 55 through the butt joint part 54, is converted in the spot size in the spot size conversion region 57, and is emitted from a light exit facet 55a.
The laser light converted in spot size is input into the light incident plane of the single mode optical fiber from the light exit facet 55a, and thereby the semiconductor optical device 51 and the single mode optical fiber are optically coupled. Patent document 1: Jpn. Pat. Appln. KOKAI Publication No. 9-61652
However, in the conventional semiconductor optical device 51 having the taper light waveguide 55 disclosed in patent document 1, the taper light waveguide 55 must be formed integrally with the semiconductor optical device 51 so that the spot size of the laser light emitted from the light exit facet 55a is matched with the spot size of the single mode optical fiber. Therefore, it is hard to design, it takes much time and labor in manufacture, and repeatability of manufacture is poor.
Incidentally, by expanding the width of the active layer of the semiconductor laser, the spot diameter of the laser light emitted from the semiconductor laser can be adjusted to the core diameter of the single mode optical fiber.
However, by merely expanding the width of the active layer of the semiconductor laser alone, a lateral harmonic mode is generated in the laser light emitted from the semiconductor laser, so that a discontinuous point is formed as regards the current-light output characteristic, and coupling loss is increased at the time of optically coupling with the single mode optical fiber, which poses a new problem.
The present inventors or the like previously disclosed, in U.S. patent application Ser. No. 10/692,125 and European patent application No. 03 025 058.3, a semiconductor light emitting device applicable to a semiconductor laser, which is capable of obtaining a light output of higher power in a simple configuration, and expanding the maximum width of an active layer that can suppress a lateral harmonic mode because an n-type cladding layer is made of InGaAsP having larger refractive index than a p-type cladding layer made of InP.
These prior applications, however, does not suppose that a semiconductor laser and a single mode optical fiber are optically coupled without using a lens in order to transmit an optical signal from the semiconductor laser as a light source as considered in the present invention, and thus, the expandable width of the active layer is 3.5 μm or more, and 4.0 μm at maximum.
Therefore, if the semiconductor light emitting device disclosed in these prior applications is applied in a semiconductor laser as a light source used in an optical communication system, the expandable width of an active layer is 3.5 μm to 4.0 μm at most. Consequently, it is not realistic from the viewpoint of coupling loss to couple the semiconductor laser and a single mode optical fiber optically without using a lens.